Twin Study: Marijuana Use Has No Direct Effect on IQ

The cumulative use of cannabis by adolescents has no direct effect on intelligence decline, according to longitudinal data published today in the Proceedings of the National Academy of Sciences.


Investigators at the University of California, Los Angeles and the University of Minnesota evaluated whether marijuana use was associated with changes in intellectual performance in two longitudinal cohorts of adolescent twins. Participants were assessed for intelligence at ages 9 to 12, before marijuana involvement, and again at ages 17 to 20.

Researchers reported no dose-response relationship between cannabis use and IQ decline. They also found no significant differences in performance among marijuana using subjects when compared to their non-using twins.

Investigators concluded: “In the largest longitudinal examination of marijuana use and IQ change, … we find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline. … [T]he lack of a dose–response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana.”

The findings follow the publication of a separate longitudinal study in the Journal of Pharmacology which concluded that cumulative adolescent marijuana use is not associated with lower IQ or poorer educational performance once adjustments are made for potential confounders, specifically cigarette smoking.

An abstract of the study, “Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies,” is available online.

This article first appeared on NORML.
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